Methods and systems for increasing reach and/or split in passive optical networks

ABSTRACT

Systems and methods according to these exemplary embodiments provide for methods and systems that allow for either reducing signal loss or improving the optical signal strength in a PON for increasing optical signal range.

TECHNICAL FIELD

The present invention relates generally to telecommunications systemsand in particular to methods and systems for increasing reach/split inpassive optical networks.

BACKGROUND

Communications technologies and uses have greatly changed over the lastfew decades. In the fairly recent past, copper wire technologies werethe primary mechanism used for transmitting voice communications overlong distances. As computers were introduced the desire to exchange databetween remote sites became desirable for many purposes such as those ofbusinesses, individual users and educational institutions. Theintroduction of cable television provided additional options forincreasing communications and data delivery from businesses to thepublic. As technology continued to move forward, digital subscriber line(DSL) transmission equipment was introduced which allowed for fasterdata transmissions over the existing copper phone wire infrastructure.Additionally, two way exchanges of information over the cableinfrastructure became available to businesses and the public. Theseadvances have promoted growth in service options available for use,which in turn increases the need to continue to improve the availablebandwidth for delivering these services, particularly as the quality ofvideo and overall amount of content available for delivery increases.

One promising technology that has been introduced is the use of opticalfibers for telecommunication purposes. Optical fiber network standards,such as synchronous optical networks (SONET) and the synchronous digitalhierarchy (SDH) over optical transport (OTN), have been in existencesince the 1980s and allow for the possibility to use the high capacityand low attenuation of optical fibers for long haul transport ofaggregated network traffic. These standards have been improved upon andtoday, using OC-768/STM-256 (versions of the SONET and SDH standardsrespectively), a line rate of 40 gigabits/second is achievable usingdense wave division multiplexing (DWDM) on standard optical fibers.

In the access domain, information regarding optical networking can befound in Ethernet in the First Mile (EFM) standards (IEEE 802.3ah whichcan be found at www.ieee802. org and is included herein by reference)supporting data transport over point-to-point (p2p) andpoint-to-multipoint (p2mp) optical fiber based access networkstructures. Additionally the International Telecommunications Union(ITU) has standards for p2mp relating to the use of optical accessnetworking. Networks of particular interest for this specification arepassive optical networks (PONs). For example, three PONs of interestare, e.g., Ethernet PONs (EPONs), broadband PONs (BPONs) and gigabitcapable PONs (GPONs), which are displayed below for comparison in Table1.

TABLE 1 Major PON Technologies and Properties Characteristics EPON BPONGPON Standard IEEE 802.3ah ITU-T G.983 ITU-T G.984 Protocol Ethernet ATMEthernet Rates (Mbps) 1244 up/1244 down 622/1244 down 1244/2488 down155/622 up 155 to 2488 up Span (Km) 10 20 20 Number 16 32 64 of Splits

An exemplary GPON 100 in FIG. 1 shows elements of an opticaldistribution network (ODN) that interact with various endpoints of anoptical network termination (ONT). Additionally GPON 100 uses wavedivision multiplexing (WDM) on the optical signals. As shown in FIG. 1,one or more service providers or types 102 can be in communication withan optical line termination (OLT) 104, which is typically located in acentral office (CO) (not shown). The OLT 104 provides the network sideinterface and is typically in communication with at least one opticalnetwork termination (ONT) (or an optical network unit (ONU) whichperforms similar duties as an ONT but typically for a multi-dwellingunit). These service providers 102 can provide a variety of servicessuch as video-on-demand or high definition television (HDTV), Voice overIP (VoIP) and high speed internet access (HSIA). The OLT 104 transmitsinformation to WDM 106 which multiplexes the data and transmits the dataoptically to a passive combiner/splitter 108. The passivecombiner/splitter 108 then splits the signal and transmits it to theupstream WDMs 110 and 116. These WDMs 110 and 116 demultiplex the signaland forward it on to their respective ONTs 112 and 118. These WDMs (108,110 and 116) are typically integrated into both the OLT and the ONTs andare used for placing and extracting the upstream and downstreamwavelengths depending upon their locations in the optical network. TheseONTs 112 and 118 then forward the information onto their respective endusers (EU) 114, 120 and 122.

It will be understood by those skilled in the art that this purelyillustrative GPON 100 can be implemented in various ways, e.g., withmodifications where different functions are combined or performed in adifferent manner. For example these WDMs (108, 110 and 116) typicallyare duplexers, but if an additional signal is being transmitted, e.g., acable-television signal in a GPON, they can act as triplexers.Additionally in the upstream direction, the optical signal wouldtypically have a different wavelength from the downstream signal and usethe same WDMs 106, 110 and 116, which have bidirectional capabilities.

With the advent of the above described services and the ongoingimprovements in optical networks, many telecommunication companies arechoosing to upgrade their copper centric access networks with fiberoptic access networks. Some such upgrades include, for example, usingone of the above described PON networks combined with fiber to the home(FTTH), and/or hybrid networks, e.g., fiber to the cabinet (FTTC)combining optical EFM and/or PON for data backhaul with very high speeddigital subscriber line (VDSL2) by reusing the last hundred meters or soof copper wire. These upgrades allow an increase in the types andquality of services delivered by companies to end users. A comparison oftwo different types of optical distributions networks (ODNs) aresummarized below in Table 2.

TABLE 2 P2P vs. P2PM P2P P2PM (GPON) Mature technology, low risk Newtechnology, higher risk Favored by non-telcos Favored by T1 (closednetwork) in open network Main markets: Northern and Main markets: US andSouthern Europe Western Europe Lowest CapEx today Low OpEx today, higherprice erosion

Regardless of which type of optical system, i.e., p2p or GPON (or both),is deployed, one of the primary requirements for low capital expenditure(CapEx) and operational expenditure (OpEx) is for the optical system toemploy a passive ODN, e.g., using only passive optical componentsbetween the central office (CO) and the user equipment (FTTH) or thecabinet (FTTC). Examples of the passive optical components includeconnectors, fibers, splices and passive power splitters (PPS). Adownside to using only passive optical components is that the overallsignal reach becomes reduced as a function of the number of splits inthe system. For example, in a typical PON which is communicating with upto 64 end users, the effective usable signal strength distance isapproximately 20 kilometers.

An acceptable amount of loss allowable attributable splitters (alsoreferred to as “splitter insertion loss”) in an ODN is specified by, forexample, the G.984.2 specification for GPONs depending upon opticalclass. For more information regarding GPONs in general, the interestedreader is directed to the G.984.1-4 standards which can be found atwww.itu.int/rec/T-REC-G/en, the disclosure of which is incorporatedherein by reference. Three general optical classes are class A optics(which allow for a loss between 5 to 20 dB), class B optics (which allowfor a loss between 10 to 25 dB) and class C optics (which allow for aloss between 15 to 30 dB). A current industry standard used for GPONs isconsidered to be a B+ optics class which allows for a maximum loss of 28dB over an ODN. In other words, the optical transceivers in an OLT andthe ONT(s) should be able to perform to provide an acceptable output onan ODN where the passive components, e.g., splices, connectors, fibersand splitters, together have an insertion loss of 28 dB. This linkbudget also typically needs to take into account other power penaltiesand some amount of system margin.

Different passive components within an ODN provide different amounts ofloss during transmission. Table 3 below shows typical ODN components andthe associated loss.

TABLE 3 Typical ODN Components and Associated Losses Component AverageLoss Description Single Mode Fiber 0.4 dB/km @ 1310 nm G.652.B 0.25dB/km @ 1550 nm Connector/Splice 0.1-0.2 dB LC/PC Type Passive 1 × 4 7.5 dB Standard Grade Power splitter/Combiner 1 × 8   11 dB Splitter 1× 16 14.2 dB 1 × 32 17.8 dB 1 × 64 21.1 dB 1 × 128 23.8 dBAs can be seen in Table 3, the splitter typically contributes thelargest amount of loss in an ODN. For example, the loss associated witha splitting ratio of 1:64 (which is a commonly desired ratio today) is21.1 dB, which roughly equates to the loss generated by passing anoptical signal through a fiber with a length of 53 km (e.g., the fiberloss at the 1310 nm wavelength for transmissions over 53 km of singlemode fiber). The fiber loss over that distance is shown in the equationbelow.53 km×0.4 dB/km=21.2 dB   (1)Looking at the optical losses due to the splitters from anotherperspective using the data in Table 3 above, for a B+ system with a 28dB link budget, a 1×64 splitter would reduce the reach (i.e., the usefultransmit distance) of the PON to 18 km. Thus, even small reductions inthe splitter insertion loss could result in appreciable increases in PONoptical signal reach or possibly the number of splits while maintaininga similar reach. For example, a doubling of the split ratio implies a +3dB increase in insertion loss which equates to approximately 7.5 km inreach.

Other link budget considerations also exist and should be addressed toextend the reach of a PON. One issue is that for EPONs and GPONs theupstream transmission structure typically has a shorter reach ascompared to the reach in the downstream transmission direction. Thecause for this difference is inherent to the time divisionmultiplexing/time division multiple access (TDM/TDMA) protocol structureused on the PONS, as will be described in more detail below. Thedivision between downlink and uplink is done via WDM where the downlinkoperates on a wavelength of 1490 nm with a bandwidth of 20 nm and theuplink operates on a wavelength of 1310 nm with a bandwidth of 100 nm.The data in the downlink is broadcasted to all ONTs in the PON using aTDM scheme where each of the ONTs takes data from its assigned timeslotin the downstream signal. The downstream optical signal is a continuouswave with equal power transmitted towards all ONTs. The optical terminaltransceiver (OTRx) located in the OLT is shared by all ONTs and thus cancontain high quality optics with a high output power.

In the upstream direction, a TDMA scheme (e.g., as shown in FIG. 2) isused where ONTs 202 and 206 are allowed to transmit data in grantedtime-slots on their optical wavelength(s). This means that ONTs 202, 206transmit in a burst mode at their allotted time slots, as compared to acontinuous power transmission in the downstream direction from the OLT210. Since the ONTs 202, 206 are located at different distances from theOLT 210, the ONTs 202, 206 are informed by the OLT 210 when, and withwhat power, to transmit their respective bursts so that the ONTs signalsare arriving in an aligned time structure at the OLT 210. For example,ONT1 202 receives the continuous transmission 212 and receives itsinformation from its assigned time slot 204. ONT2 206 performs similarfunctions and receives its information from timeslot 208. Based on thereceived data the ONTs know their transmission time slot which resultsin an upstream message 214 where the different ONT outputs are in a timesequential order.

Given this TDMA approach, the OLT 210 typically includes a burstreceiver that decodes the ONTs data which arrives slightly jittered (orasynchronous) with differing power levels. At higher data rates oftransmission this decoding process becomes more challenging to perform.For example, currently systems operating at transmission rates of 1.25Gbit/s are considered to be cost efficient, transmission rates on theorder of 2.5 Gbit/s are considered to be technically feasible, whiletransmission rates of 5-10 Gbit/s are not currently considered feasiblein this type of optical communication system. This leads GPONs to havean asymmetric data rate. The use of a burst receiver introduces a burstpenalty in the area of 3-6 dB depending upon the quality of thecomponents in the OTRx. Coupling this burst penalty with a slightlyhigher loss on the upstream band (approximately 0.15 dB/km) and the needto use less expensive optical components (diplexer, triplexer) in theONTs due to scalability reasons, transmission in the upstream directionbecomes the limiting direction for this type of optical system.

To overcome this challenge of obtaining a greater transmit distance witha usable optical signal, while also maintaining a high number ofallowable splits, different possible solutions can be considered.Generally, either the losses introduced by the splitters need to bereduced, the signal needs to be amplified or both. Regarding thepossible solution of amplification, a variety of options exist asillustrated generally in FIG. 3. Therein, three potential locations foradding a booster for amplification are the CO 302, a remote node 304, orwith each ONT 310 and 312 at a location such as home1 306 and home2 308.The booster in the CO 302 is shown as booster 314 near the OLT 316, thebooster in the remote node 304 is shown as booster 318 downstream of thepassive combiner splitter 320 and in the homes (or near the ONTs) theboosters are shown as boosters 322. Putting a booster or amplifier inany of these locations brings with it different, associated problems.For example, it would be cost prohibitive for a booster 322 to belocated with each ONT 310 and 312 due to the high numbers of ONTs in asystem. If a booster 318 were to be put in a remote node 304, it wouldadd a need for power and perhaps more maintenance visits, to anotherwise passive location. Regarding placing a booster 314 in the CO302 near the OLT 316, this also is not unproblematic since the booster314 can only be operated in a low power mode due to non-linearities onthe fiber. Moreover, since a typical GPON system is upstream limited andthe input sensitivity of a pre-amplifier in the OLT booster 314 isapproximately −28 dBm, this solution would be an added expense with novalue for the upstream signal.

Accordingly the exemplary embodiments described herein provide systemsand methods that allow, e.g., for either reducing signal loss orimproving the optical signal strength in a PON.

SUMMARY

Systems and methods according to the present invention address this needand others by reducing signal loss or improving the optical signalstrength in a passive optical network (PON).

According to one exemplary embodiment a method for opticalcommunications includes the steps of: receiving a plurality of opticalsignals; coupling the plurality of optical signals at a light switchingunit by switching between the plurality of optical signals into asequentially combined signal onto an output fiber; and transmitting thecombined signal,

According to another exemplary embodiment a method for equalizingoptical signals includes the steps of: receiving schedule informationfor each of a plurality of optical signals, wherein the scheduleinformation includes at least timing and power information associatedwith the plurality of optical signals; and adjusting each of the opticalsignals by either amplifying or attenuating each of the plurality ofoptical signals based on the schedule information.

According to another exemplary embodiment a method for opticalcommunications including the steps of receiving, at an optical linetermination, a plurality of optical signals from a plurality of upstreamlocations at an optical power equalizer within the optical linetermination; equalizing the plurality of optical signals from theplurality of upstream locations at the power equalizer, wherein the stepof equalizing further includes: splitting off a portion of each of theplurality of optical signals; measuring an amplitude for each portion ofthe plurality of optical signals; and adapting each of said plurality ofoptical signals based on a corresponding, measured amplitude by eitheramplifying or attenuating each of the plurality of optical signals togenerate equalized signals; and transmitting the equalized signals to areceiving unit within the optical line termination.

According to yet another exemplary embodiment a node for opticalcommunications comprising: at least one port for receiving a pluralityof optical signals; a light switching unit which couples the pluralityof optical signals by switching between the plurality of optical signalsinto a sequentially combined signal onto an output fiber; and aninterface for transmitting the combined signal.

According to yet another exemplary embodiment a node for equalizingoptical signals comprising: a scheduler for receiving scheduleinformation for each of a plurality of optical signals, wherein theschedule information includes at least timing and power informationassociated with the plurality of optical signals; and an adjuster foradjusting each of the plurality of optical signals by either amplifyingor attenuating each of the plurality of optical signals based upon theschedule information.

According to yet another exemplary embodiment a node for opticalcommunications comprising: an optical line termination for receiving aplurality of optical signals from a plurality of upstream locations atan optical power equalizer within the optical line termination; anoptical equalizer for equalizing optical powers associated with theplurality of optical signals from the plurality of upstream locations,the optical equalizer including: at least one tapped coupler forsplitting off a portion of each of the plurality of optical signals; andat least one sensor for measuring an amplitude of each portion of theplurality of optical signals; wherein each of the plurality of opticalsignals is adapted based on a corresponding, measured amplitude byeither amplifying or attenuating each of the plurality of opticalsignals; and the equalized signals are transmitted to a receiving unitwithin the optical line termination.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate exemplary embodiments, wherein:

FIG. 1 depicts a Gigabit Passive Optical Network (GPON);

FIG. 2 illustrates Optical Network Terminations (ONTs) using a timedivision multiple access (TDMA) scheme;

FIG. 3 illustrates potential booster placement options in a PON;

FIG. 4 shows a 1:N passive power splitter (PPS);

FIG. 5 depicts a pseudo-passive power splitter (PPPS) structureaccording to exemplary embodiments;

FIGS. 6( a)-(b) show signals associated with a control and switchingunit (CSU) according to exemplary embodiments;

FIG. 7 shows functions applied to generate output signals from the CSUaccording to exemplary embodiments;

FIG. 8 illustrates an upstream timing diagram according to exemplaryembodiments;

FIG. 9( a) illustrates a first light path in a light switching unit(LSU) according to exemplary embodiments;

FIG. 9( b) shows a second light path in an LSU according to exemplaryembodiments;

FIG. 10 illustrates receiving two light streams and their associatedpaths in an LSU according to exemplary embodiments;

FIG. 11 shows a power equalizer (PE) in an OLT according to exemplaryembodiments;

FIG. 12 shows a PE remotely located between an OLT and passive powersplitter;

FIG. 13 shows a method flowchart for increasing signal reach accordingto exemplary embodiments;

FIG. 14 shows a method flowchart for reducing signal loss according toexemplary embodiments;

FIG. 15 depicts a method flowchart for increasing transmission distanceaccording to exemplary embodiments;

FIG. 16 shows a method flowchart for increasing transmission distanceaccording to exemplary embodiments; and

FIG. 17 depicts a method flowchart for increasing optical signal rangeaccording to exemplary embodiments.

DETAILED DESCRIPTION

The following detailed description of the exemplary embodiments refersto the accompanying drawings. The same reference numbers in differentdrawings identify the same or similar elements. Also, the followingdetailed description does not limit the invention. Instead, the scope ofthe invention is defined by the appended claims.

As mentioned above, it is desirable to provide mechanisms and methodsthat allow for reducing signal loss, improving signal strength in apassive optical network (PON), or both, albeit the present invention isnot limited thereto as will be described below. As described above inthe Background, a typical PON (be it a Gigabit-capable PON (GPON),broadband PON (BPON) or an Ethernet PON (EPON)) includes some type ofsplitter which is a source of transmission loss. Different passive powersplitters (PPS) are commercially available today such as the fusedbiconical taper (FBT) splitter or the planar lightwave circuit (PLC)splitter. Technical approaches using either type of PPS can result inPONs with substantially the same transmission characteristics.

In a PON, an optical splitter typically divides the power equally amongthe signals in a 1:N ratio. FIG. 4 shows a 1:N PPS splitter 400 whereinan optical signal 401 enters the splitter and is output as two opticalsignals of equal strength 402 and 403. These two optical signals arethen in turn split by presenting them to subsequent splitter ports, eachtime equally dividing the signal strength between two optical signals.As can be seen in FIG. 4, one signal becomes two signals, two signalsbecome four signals and four signals become eight signals out to Nsignals. The theoretical insertion loss attributable to these splitterscan be calculated by, for example, using the following equation:Loss=−10 log(N) dB   (2)This means that the loss increases by approximately 3 dB per doubling ofsplitter ports. Additionally, these PPSs 400 are passive (i.e., they donot increase the gain of the optical signal) and reciprocal which meansthat the PPS can be used in both directions (the PPS can act as acombiner) but with the same insertion loss. Thus in the case of a GPONusing a PPS, light traveling from the ONT direction (upstream) to theOLT experiences the full insertion loss of −10 log(N) dB.Pseudo-Passive Power Splitter

According to exemplary embodiments a pseudo-passive power splitter(PPPS) 500 structure as shown in FIG. 5 can be used to minimize splitterinsertion loss, as well as to provide other benefits for improvingusable optical signal strength distance within a PON. The PPPS 500includes two optical data signal paths (upstream and downstream), apower path (light and electrical) and a control path. As shown by thearrows in FIG. 5, optical signals passing through the PPPS 500 from anOLT side 502 toward an ONT side 528 will be referred to herein as“downstream” or “downlink” optical signals. Conversely, optical signalspassing through the PPPS 500 in the opposite direction (towards the OLTside 502) will be referred to herein as “upstream” or “uplink” opticalsignals. For signals entering or leaving the PPPS 500, regularwavelength multiplexers such as low loss thin-film filters (TFFs) 504(upstream side), 506, 508 and 510 (downstream side) are used toseparate/combine uplink, downlink and pump wavelengths as needed.Alternatively, instead of using a TFF 504, 506, 508 and 510 an arrayedwavelength grating (AWG) could be used. In the downstream path, opticalsignals are received from the OLT 502 side by TFF 504. TFF 504 canreceive at least two downstream optical signals on two differentwavelengths or wavelength ranges. One downstream optical signal includesthe downstream data for the various ONTs which can be transmitted on oneor more optical wavelengths reserved for data transmission, and thesecond downstream optical signal (which is light power 516) can betransmitted on a different wavelength, i.e., different from thedownstream and upstream data wavelengths used by the PON. The downstreamwavelength is then forwarded to an optional amplifier 512 (which couldbe, for example, either a semiconductor optical amplifier (SOA), a fiberamplifier, such as, a phosphorous doped fiber amplifier (PDFA) or aRaman amplifier) for amplification before the downstream optical signal,which is a high-rate continuous-wave signal, is split in a 1:N PPS 514.

If optional amplifier 512 is present and used, power needs to beprovided to the optional amplifier 512. Light power 516 is forwardedfrom the TFF 504 to a power pump unit (PPU) 518. This light power 516can be used in different ways to power different devices within the PPPS500 as necessary. For example, if optional amplifier 512 is a PDFA orRaman amplifier, light power 516 can be pumped through power unit 518 tothe PDFA optional amplifier 512 for power purposes. If optionalamplifier 512 is an SOA, then light power 516 can be converted toelectricity in power conversion unit (PCU) 520 and the electrical poweris sent to the SOA optional amplifier 512 for use. The light power 516can, for example, be converted into electricity via photovoltaicelements with a conversion efficiency up to approximately 80%. In mostexemplary embodiments, depending upon the pump laser and the wavelengthused, at least 1 W of electronic power can be generated from lighttransmitted over a standard single mode fiber with a reach of between 10to 20 km.

Additionally other devices that may be within the PPPS 500 receive powerfrom units 518 and 520 as needed. For example other amplifiers, such asa variable optical amplifier (VOA) 522 or a thorium doped fiberamplifier (TDFA) can receive either light power or electrical powerdepending upon what type of amplifier is used and electrical power issent to the control and switching unit (CSU) 524. The exemplaryembodiments described above with respect to powering the PPPS 500 shownin FIG. 5 can also be modified such that the light power is receivedfrom the ONT side 528 instead of the OLT 502 side. For example, in asystem with 64 splits allowing for 64 ONTs, while one ONT istransmitting data, the other 63 ONTs can transmit light for power. Thiscan be seen in FIG. 5, where light power 538 is being forwarded from LSU526 to the PPU 518 for power purposes. As can be seen, the moreallowable splits, the more ONTs, the higher the possible power receivedfor use at the PPPS 500.

After optical amplification, if it occurs, at optional amplifier 512,the optical signal is sent on to the PPS 514. The PPS 514 splits thereceived optical signal into N signals for each of the N downstream sideTFFs as shown by TFF1 506, TFF2 508 and TFFn 510. This optical signalsplitting is typically performed in a manner as described above withrespect to FIG. 4. These TFFs (506, 508 and 510) then forward theoptical signals to their associated ONTs.

According to exemplary embodiments, the PPPS 500 is capable of acting asa combiner in the upstream direction. Optical signals are received fromthe 1 to N ONTs (represented by ONT side 528) associated with the GPON.These optical signals are received according to their predetermined timeslots at their associated TFFs (TFF1 506, TFF2 508 and TFFn 510). FromTFF1 506 a received optical signal is forwarded to its associated tappedcoupler unit1 (TCU1) 530, with the TCU1 530 having, for example, a lowtap ratio of approximately 1-5% of signal power. TCU1 530 sends thereceived optical signal to the light switching unit 526 and an on/offcontrol signal to the control switching unit 524. The LSU 526 alsoreceives optical signals from the other TCUs, e.g., TCU2 532 throughTCUn 534. The CSU 524 also receives on/off control signals from theother TCUs, e.g., TCU2 532 through TCUn 534. From the received controlsignals, the control logic within the CSU 524 instructs the LSU 526 whento rotate mirrors (not shown in FIG. 5, described below) within it toreceive the appropriate optical signal. In this manner the LSU 526creates a combined signal in a substantially lossless manner fortransmission upstream from all of the downstream ONTs. More detailsregarding the CSU 524 and the LSU 526 are provided below.

The combined signal is sent from the LSU 526 to an optional amplifier522 (if present) such as a variable optical amplifier (VOA) foramplification. Alternatively, if power is a constraint or it isdesirable to keep part of the system passive, a variable opticalattenuator could be used in the place of optional amplifier 522.Additionally, depending upon the need and/or type of amplifier used asoptional amplifier 522, equalization and/or attenuation can also beperformed as also discussed in more detail below. To facilitate thisamplification the optional amplifier 522 receives appropriate power asneeded (in a manner similar to that described above for optionalamplifier 512) and a control signal from the CSU 524 for setting up thedesired attenuation. From the optional amplifier 522, the uplinkwavelength is then sent to the TFF 504 for transmission upstream to theOLT side 502.

As described above, the operation of the CSU 524 and the LSU 526 aids inreducing the optical signal loss in the upstream direction. According toexemplary embodiments, the CSU 524 performs two tasks. Namely, the CSU524 sends timing signals to the LSU 526 and control signals to theoptional amplifier 522 (if present). The signals associated with CSU 524according to an exemplary embodiment are illustrated in FIGS. 6( a)-(b)and include input signals from the TCUs 530, 532 and 534, an outputsignal S 602 and an output signal A 604. Input signals (also referred toherein as control on/off signals) are received from the various TCUs,for example, L_1 606 is received from TCU1 530 and L_N 608 is receivedfrom TCUn 534. These input signals include information regarding thetiming relating to the different ONT transmission intervals andinformation about the transmission power of the received signals fromthe different ONTs. Examples of these input signals can be seen insignals 610, 612 and 614, where different power levels and timeintervals are shown for each of the signals 610, 612 and 614.

Based on these received inputs, a timing signal S 602 is sent from CSU524 to the LSU 526. The S signal 602 as shown in signal 616 shows anumber of peaks. Each peak in signal 616 indicates to the LSU 526 toswitch the light path for the next ONT signal to be received. Forexample, peak 620 could indicate to switch to the light path associatedwith the ONT signal coming from TCU2 508. The A signal 604 is used, forexample, to set up the amplification (or attenuation depending uponwhether an amplifier or an attenuator is used as optional amplifier 522)for optional amplifier 522 and includes timing and power levelsassociated with the combined optical signal that is transmitted upstreamfrom LSU 526 as shown in signal 618.

These exemplary signals can be further described using the diagram ofthe CSU 524 function as shown in FIG. 7. The optical input signals L_1606 through L_N 608 are received at CSU 524 and undergo an optical toelectrical conversion 702. These converted signals are sent forprocessing by a function generator 704 which generates the sum(performed in the electronic domain) and transmits the sum as twooutputs. Output signal A 604 can be sent to the optional amplifier 522without further processing, but the other output signal has adifferentiation of the sum performed over time (d/dt) 706 prior tobecoming output signal S 602 and transmitted to the LSU 526.

The timing involved with the received signals which leads to, e.g., themirror switching within LSU 526 can, for example, be performed inaccordance with the timing constants as described in G.984.1-4 forGPONs. For example, G984.2 specifies the physical overhead time (Tplo)that is precedent to any upstream burst. This overhead time is used forfive physical processes in the GPON which are as follows: (1) laseron/off time, (2) timing drift tolerance, (3) level recovery, (4) clockrecovery and (5) start of burst delimitation. Timing windows arespecified for relevant upstream rates as shown below in Table 4.

TABLE 4 Time Windows for Upstream Rates Upstream Rate (Mbit/s) OverheadBytes (Bytes) Corresponding Time 1244.16 (actual) 12 (96 bits) 77 ns2488.43 (future) 24 (192 bits) 77 ns

FIG. 8 shows an exemplary upstream timing diagram associated withupstream optical signals which can pass through PPPS 500. The upstreamtiming diagram illustrates the types of upstream data sent, the relativetiming sequence and the number of bits allowed for each field ofupstream data based upon the two different transmission rates. The guardtime (Tg) 802 and the laser on/off times, account for 32 bits with atransmission rate of 1.25 Gbit/s and 64 bits for a transmission rate of2.5 Gbit/s. The corresponding time, Tg 802, is currently 25.7 ns and, assuch, the guard time Tg 802 is sufficiently long to allow the mirrors inthe LSU 526 to be switched and for setting up the optional amplifier 522(or optional optical attenuator). The guard time could be even longerdepending upon optical system implementations. For example, adding reachextender boxes to the system would require extending the guard band toat least 72 bits which corresponds to a time of 57 ns. Moreover it canbe seen from FIG. 8 that the minimum length of an upstream burst isgiven by a burst containing only physical layer overhead (PLOu) at arate of 1.25 Gbit/s, which in total contains 44 bits of preamble, 20bits of delimiter, 1 byte of interleaved parity (BIP), 1 byte of ONU-IDand 1 byte of status report. For a transmission rate of 2.5 Gbit/s thePLOu in total contains 108 bits of preamble, 20 bits of delimiter, 1byte of interleaved parity (BIP), 1 byte of ONU-ID and 1 byte of statusreport. Converting this data block to a time reference, the minimumburst would thus last for 70.6 ns at a transmission rate of 1.25 Gbit/sand 61 ns at a transmission rate of 2.5 Gbit/s. Thus, at a minimum, themirrors in the LSU 526 will remain in a single position without needingto be switched for 61 ns. This, in turn, means that the minimum periodfor the LSU 526 switching signal S 602 should be on the order of the sumof the time periods for Tg 802 plus the time associated with the PLOu,which is approximately 87 ns or 11.49 MHz. Therefore, according toexemplary embodiments, a CSU 524 should generate the S signal 602 andthe A signal 604, e.g., at a rate greater than 11.49 MHz.

Using the above described exemplary CSU 524 and associated signals andcontrol logic, an exemplary LSU 526 will now be described. The LSU 526can, for example, be implemented as a mechanical time-domain device thatis non-reciprocal and thus allows for basically lossless switching oflight paths. FIGS. 9( a)-9(b) show an exemplary 2:1 device where arotating mirror 900 switches the light from different input fibers tothe output fiber 902. Initially, in FIG. 9( a), light is coming in frominput fiber1 904 and goes into the mirror unit 908 and is directed outto the output fiber 902. The signal from input fiber1 904 is thencompleted and mirror unit 908 changes its position, e.g., to theposition indicated as a dashed line. This rotation of the mirror unit908 can be performed in a step-wise manner during the ONT transmissionwindows, e.g., gap period (as shown by TX off time 808 in FIG. 8), undercontrol of the CSU 524 through the S signal 602. At this time mirrorunit 908 is ready to receive an optical signal from input fiber2 906. InFIG. 9( b) it can be seen that mirror unit 908 has been rotated, is nowreceiving an optical signal from input fiber2 906 and is directing thatoptical signal to the output fiber 902. While the LSU 526 shown in FIG.9 is a 2:1 switching device, it should be understood that the LSU 526can be more generally implemented as an N:1 switching device with Nbeing a power of 2 and the N devices being used in a similar manner tothat shown with respect to the splitters in FIG. 4.

For GPON uplinks, a time of, for example, 25 ns is available toflip/rotate the mirror in the LSU 526. With a mode diameter greaterthan, for example, 9.72 μm for single mode transmission and assuming 95%of the power is disposed in 5× the mode radius in the cladding, thedimension of the mirror can, for example, be approximately 36 μm. Thissize is well within the current capabilities of silicon fabricationtechniques, e.g., where 0.9 μm sized items of technology can be placedon silicon today. According to one exemplary embodiment, a silicon chipstyle LSU 526 using optical MEMS mirrors will have an insertion loss ofless than 1 dB and a switching time of, e.g., 25 ns. However, differentcombinations of switch time and loss can be used as desired,particularly when influenced by potential variables such as, cost,changes in upstream timing, changes to standards and ease of devicemanufacturing. Another exemplary switch technology which can be used toimplement LSU 526 operates on the vertical coupler switch (VCS)/SOAprinciple and has a low switch time, e.g., under 1.5 ns, and a low loss,approximately 0 dB. The VCS/SOA principle can be generally described asdividing the wavepath in a vertically aligned resonator that isintroducing an amplification. Thus, the signal split is compensated by asmall amplification in the resonator. Still other switching techniquescan also be used to implement LSU 526. For example, a variety of otheroptical space switch technologies, shown in Table 5 below, can be usedto fabricate LSU 526 such as, electro-optical switches based on aMach-Zehnder Interferometer (MZI), SOA or VCS as well as holographicswitches, Micro-Electro-Mechanical System (MEMS) micro mirror devicescalled Optical MEMS (OMM) and electro-optical beam deflector technologycan also perform the switching function in LSU 526.

TABLE 5 Optical Space Switches Cross- Switch Switch Loss talk PDL Power/Ref. Material Principle Effect I × O Time (dB) (dB) (dB) Voltage EO MZIInP MZI EO 4 × 4 200 ps <6 <−13 <1 4.5 V substrate/ (F-F) bulk InGaAsPDOS SOA InP VCS/SOA Carrier 4 × 4 <1.5 ns 0 −50 5 V VCS substrate/Injection (F-F) bulk InGaAsP DOS SOA InP VCS/SOA w/λ- Carrier 4 × 4 <1.5ns 0 −9 5 V VCS w/λ- substrate/ conversion injection (F-F) conv bulk (at10 Gbit/s) and XGM InGaAsP EO Y- PZT/PLZT/ Router -Selector EO 8 × 8 ~20ns ~5 −40 10 V branch (Nb:ST) architecture substrate DOS SC Y- InP/Router -Selector Carrier 8 × 8 <100 ns 25 −13 −3 340 mW branch InGaAsParchitecture by Injection hybrid assembly HOLOGR KLTN Double-stage 240 ×240 30 ns ~4 <300 W crystal Electroholography EO Beam AlGaAs/ WaveguideArray EO phase 64 × 64 <20 ns ~15 −19.5 Deflectors GaAs Deflectorsmodulation Ceramic (free space substrate device)

According to another exemplary embodiment, the LSU 526 can be powered bythe lasers from the ONT side that are not currently transmitting anoptical data signal. This exemplary feature will now be described withrespect to FIG. 10. FIG. 10 shows an output fiber data 1002 that iscurrently receiving an optical signal 1012 from input fiber1 1008 basedupon the mirror unit's 1010 current position. Input fiber2 1006 isreceiving an optical signal 1010 from a first ONT which is beingdirected to output fiber power 1004. In a PON environment typically asingle ONT is transmitting at a time as discussed above with respect toFIG. 2. However in this exemplary embodiment, the other ONTs' lasers aretransmitting light for power on the same wavelength as the ONT which istransmitting data. The CSU 524 controls the mirror unit 1014 in LSU 526such that the correct data is being received and the other receivedoptical signal(s) are being forwarded to the correct destination, i.e.,as light power 538 to the PPU 518. Additionally, the LSU 526 couldreceive power in a manner similar to that described above for theoptional amplifier 522 or the CSU 524 as desired.

After the optical signal is combined in LSU 526, it is passed on tooptional amplifier 522. Optional amplifier 522 is typically a variableoptical amplifier (VOA) (or variable optical attenuator) which is usedto amplify or attenuate signal amplitude as needed. The variance inamplitude typically occurs because different ONTs are located atdifferent distances from the PPPS 500. A VOA 522 thus amplifies orattenuates the signals to have substantially similar amplitude based onthe received control signal A 604 from CSU 524, prior to sending theoptical signals on to TFF 504 for transmission to the OLT side 502. Thisuse of a VOA 522 provides the further benefit of allowing the upstreamOLT to use a fixed decision interval for determining whether a signal isvalid or not, which can reduce the burst penalty.

According to another exemplary embodiment, PPPS 500 can act as a smartcoupler and perform some of the tasks and functions traditionallyperformed by an OLT. For example, logic can be stored in a memory (notshown) within PPPS 500 which, in conjunction with processingcapabilities, can perform such tasks as optical supervision, e.g.,monitoring and reporting fiber damage and breaks, protocol terminationallowing for interoperability between different PONs, as well as varioussecurity functions. Additionally, optical time domain reflectometry(OTDR) can be performed by this exemplary smart coupler to perform animproved mapping of the optical network which could provide betterinformation for matching ONTs to their specific distances from thecoupler. The memory and processing functions can be added to the PPPS500 as separate entities, or they can be combined with the capabilitiesof the CSU 524.

It will be appreciated by those skilled in the art that, according tothe above described exemplary embodiments, methods and systems fordecreasing insertion loss in PONS have been presented. This reduction ininsertion loss can be used to provide a greater reach for opticalsignals in a PON. Alternatively, or in conjunction with an increase insignal reach in a PON, this decrease in insertion loss can provide theopportunity to increase the number of splits in the PON from, e.g., atypical number of splits used in today's GPONs, e.g., 64, to a muchlarger number of splits, e.g., 512, 1024, 2048 or more, depending upondesired reach and the actual insertion loss reduction obtained for aparticular implementation. The following discussion of power equalizersprovides other exemplary techniques for reducing insertion loss in aPON.

Power Equalizer

As described above, inserting a PPPS 500 into a PON can reduce opticalsignal loss and increase optical signal reach. As mentioned in theBackground, another alternative to increasing signal reach is to amplifythe signal in such a way as to avoid some of the problems associatedwith boosters. Accordingly, other exemplary embodiments include using apower equalizer (PE) in a PON. According to different exemplaryembodiments, different types of power equalizers can be placed indifferent locations within a PON to increase optical signal reach aswill be described below.

Power equalizers can generally be described as devices that equalizeoptical signals through amplitude adjustment typically by applying gainor attenuation. These power equalizers typically receive power from apower source to perform these adjustments and also typically receiveinformation regarding the optical signals in order to make thoseadjustments. One example of a power equalizer is described in the Lee etal. patent application publication number US 2004/0247246 A1 filed onOct. 23, 2003, the disclosure of which is incorporated herein byreference. In the Lee et al. (hereinafter “Lee”) publication, a powerequalizer is placed between the passive optical splitter and the OLT ina PON. One example of a power equalizer described in Lee uses an SOA inconjunction with an active gain control circuit and a delay elementwhich modifies the optical signal based upon incoming measured opticalsignal amplitudes. While the power equalizer of Lee modifies signalstrength based upon received determined amplitudes, other types of powerequalizers (as well as other placement locations for optical powerequalizers within a system) potentially offer other benefits of interestin PONs as will be described below.

PEs of particular interest according to these exemplary embodiments canbe broken down into the two general categories herein: adaptive orscheduled. In an adaptive type of PE, the modifications to the receivedoptical signals are based upon measurements of the amplitudes of thereceived optical signals. One method for determining the receivedoptical signal power is to split out (tap) a small portion of thereceived optical power to a PIN diode (a diode with an undopedsemiconductor region between a p-type and an n-type semiconductorregion) for measurement. Additionally, in an adaptive type of PE, aprocessing delay often needs to be accounted for while the systemdetermines the amplitude from the tapped signal and adjusts the PEaccordingly to equalize the received optical signals. Also bycomparison, an adaptive type of PE tends to be fast, or react quickly,while a scheduled type of PE can be slower and tends to be less costlythan an adaptive type of PE.

In a scheduled type of PE according to exemplary embodiments, a systemscheduler is in communication with the PE. The system scheduler knowswhen signals from the various ONTs are received by the OLT and knows atwhat the optical power(s) the upstream ONTs are transmitting. The systemscheduler gains this knowledge during a “ranging procedure” which occursduring ONT start-up. This knowledge allows the scheduler to adjust thePE in time for the arrival of a new ONT signal.

Regarding the ranging procedure, the main purpose of the rangingprocedure is for the ONT to synchronize its time relative the OLT bymeasuring the round-trip delay. The OLT knows the (newly installed) ONTsignal power from the first signal sent, (regardless of message type),by measuring the average optical power the same way two end-pointdevices on a point-to-point system, which do not practice ranging, knowtheir respective signal powers. In our exemplary PON, the first signalfrom the ONT is part of the ranging process. So the OLT scheduler knowshow much each ONT signal should be attenuated for all of them to havethe same, or substantially the same, power level after processing by thePE. Also, the OLT scheduler determines the subsequent datacommunications scheduling after the ranging, and has thus allinformation on the time when each signal from the ONTs arrive at theOLT.

Within these two general types of PEs, i.e., adaptive and scheduled, twosub categories can be defined: those which apply gain to the receivedoptical signals and those which do not. This allows PEs to be classifiedinto four types as shown below in Table 7. Additionally, an example ofdifferent types of techniques/technology which can be used to implementeach type of PE is shown in Table 7, however these examples are notintended to be exhaustive.

TABLE 7 Sample PE Matrix Without Gain With Gain Adaptive Opticalmodulator with Gain saturated SOA power monitor ScheduledMicro-mechanical optical Parametric (Kerr effect) attenuatoramplification with variable pump laserThese four types of PEs allow for different methods of improving opticalsignal reach, e.g., in the upstream direction, by placing them in a PON.Additionally options exist as to where in a PON to place a powerequalizer, which further increases a PE's flexibility as will bedescribed below.

According to exemplary embodiments any of the different types of PEsdescribed above can be located in an OTRx card 1102 within an OLTchassis 1104 as illustrated in FIG. 11. The signal from the ONTs isreceived by OTRx card 1102 and initially is passed to a diplexer filter(DP) 1104 disposed thereon. Signals are separated and sent to the PE1106 which performs equalization as previously described. PE 1106 thenforwards the equalized signal to the PIN 1108 which performs the opticalto electrical conversion. If the PE 1106 is of the scheduled type, asshown in FIG. 11, system scheduler 1110 will control PE 1106. If the PE1106 is an adaptive type of PE, the other desired components, e.g., again control circuit and delay element, while not shown in FIG. 11 willbe present as needed. Alternatively, some components could be combinedsuch as the PIN 1108 and the transmitter (Tx) 1112. Additionally,signals 1114 and 1116 illustrate relative optical strengths for signalsfrom the ONTs before equalization and after equalization, respectively.

According to other exemplary embodiments, the PE 1204 can be locatedseparately from the OLT 1202, e.g., in its own location between the OLT1202 and a PPS 1206 as shown in FIG. 12. In these exemplary embodiments,each of the four general types of PEs 1206, as shown in Table 7 above,could be the PE 1206 of choice depending upon factors related to the PONin which the PE would be placed. For example, considering the case of ascheduled, passive PE a system designer could consider the followingoperational characteristics and match them to other factors influencinga PON design such as length and cost. Firstly the scheduled, passive PEcan be slower to set a new equalization (attenuation) level, as thescheduler adds control (overhead) signaling in advance before each newsignal arrives, and the scheduled, passive PE can therefore be lessadvanced and therefore less costly than an adaptive/gain device.Secondly, the scheduled, passive PE can be much wider in operatingwavelength meaning that the whole spectrum of the fiber can be availablefor use. Thus it can be used to support other future growth, e.g.,different wavelengths and multi-signals. Thirdly, the scheduled, passivePE has no limitation in bitrates as typical SOAs currently do. Andfourthly, in the case of multi-wavelength signals, the passive devicewould not impose cross talk between the signals as a SOA may do.

Additionally, other devices can be either part of the PE 1204 or inclose communication with PE 1204. For example, power may need to besupplied to PE 1204. If PE 1204 is an adaptive PE 1204, then a gaincontrol circuit and delay element may be included. If the PE 1204 is ascheduled PE, then a mechanism for communicating the timing schedule andONT output powers to the PE 1204 can be included. This latter mechanismcould be implemented in a variety of ways such as, for example,providing the PE 1204 the ability to snoop the scheduling informationfrom optical signals containing the so-called bandwidth map (i.e. theinformation to the ONTs from the OLT scheduler on when the ONTs can sendtheir next frame of information) between OLT 1202 and the ONTs active inthe PON as well as memory to store the information.

According to another exemplary embodiment a power equalizer can beplaced in PPPS 500 in place of optional amplifier 522. Morespecifically, the PEs with gain can provide additional benefits by beingplaced closer to the ONTs, e.g., as the noise is inversely proportionalto the signal power, having higher input signal powers to thegain-element as would be the case closer to the ONTs, adds less noise tothe signal. This applies for both the adaptive gain PE, e.g., a gainsaturated SOA, and for the scheduled gain PE, e.g., a parametric (Kerreffect) amplification with variable pump laser. For the gain saturatedPE to replace the optional amplifier 522 few or no changes would beneeded to the above-described exemplary PPPS 500 because the outputpower is fixed and the ability to power the gain saturated PE in thePPPS 500 is already present as previously described for powering anoptional amplifier 522. For the parametric amplification with variablepump laser minimal changes associated with controlling the PE would bemade to the PPPS 500. For example, the schedule information regardingONT timing and output power information needs to be obtained by the PPPS500 to appropriately control the PE. This could happen by, for example,giving the PPPS 500 the ability to snoop the information regardingupstream bandwidth allocation (bandwidth map), which the OLT sendsdownstream within each frame. This would allow a PPPS 500 with ascheduled PE to amplify/attenuate signals as needed in a manner similarto that described above where the PE is located within the OLT and is incommunications with a system scheduler. Alternatively this informationcould be transmitted to the PPPS 500 by the OLT. Using a scheduled PE inPPPS 500 could provide various benefits, such as, reducing/removing theneed to have the TCUs (530, 532 and 534) present (since the need to tapthe signal and determine incoming signal strength no longer exists). Thecontrol signals S 602 and A 604 from CSU 524 would then be determinedbased upon information received by snooping or information directlytransmitted to the PPPS 500 from the OLT scheduler 1110.

Note that in some exemplary embodiments, e.g., wherein the PE is placedin an OLT, the power equalizer does not have or use an external couplersince the PE 1106 is placed within the OLT 1104 as shown in FIG. 11.Also, for the scheduled PEs in either the OLT 1104 or the PPPS 500, theneed for a delay line is removed since the different signal levels fromthe ONTs are known ahead of time which allows for the appropriatecontrol signals to be sent to the PE in time to plan and execute theequalization action.

Additional benefits can accrue by using a PE in certain circumstances.For example, a PE can improve receiver sensitivity by reducing oreliminating the need for complex electronics in the receiver, which canreduce receiver sensitivity typically by 3 dB or more, to handle thevarying optical power levels particularly in high bit rate applications.This increased sensitivity enables longer reach in the upstreamdirection which is normally the limiting direction. Additionally, whenthe PE includes a gain function overall system reach can be furtherincreased.

Utilizing the above-described exemplary systems according to exemplaryembodiments, a method for increasing signal reach is shown in theflowchart of FIG. 13. Initially a method for increasing signal reachusing a coupler in a fiber optic communications network includes thesteps of: receiving, at a plurality of upstream multiplexers disposed inthe coupler, a plurality of optical signals from a plurality of upstreamlocations at step 1302; coupling the plurality of optical signals at alight switching unit into a sequentially combined signal onto an outputfiber at step 1304; and transmitting the combined signal at step 1306.

Utilizing the above-described exemplary systems according to exemplaryembodiments, a method for reducing signal loss is shown in the flowchartof FIG. 14. Initially a method for reducing signal loss in a coupler ina fiber optic communications network includes the steps of receiving anoptical signal at a multiplexer on a first fiber at step 1402; receivinga light beam at the multiplexer on a second fiber at step 1404;forwarding the optical signal to an amplifier at step 1406; amplifyingthe optical signal and forwarding the optical signal to a passive powersplitter at step 1408; splitting the optical signal at the passive powersplitter at step 1410; and forwarding the optical signal to a pluralityof multiplexers for upstream transmission at step 1412.

Utilizing the above-described exemplary systems according to exemplaryembodiments, a method for increasing transmission distance is shown inthe flowchart of FIG. 15. Initially a method for increasing transmissiondistance in a fiber optic communications network includes the steps ofreceiving, at an optical line termination, a plurality of opticalsignals from a plurality of upstream locations at a power equalizerwithin the optical line termination at step 1502; equalizing theplurality of optical signals from the plurality of upstream locations atthe power equalizer at step 1504; and transmitting the equalized signalsto a receiving unit within the optical line termination at step 1506.

Utilizing the above-described exemplary systems according to exemplaryembodiments, a method for increasing transmission distance is shown inthe flowchart of FIG. 16. Initially a method for increasing transmissiondistance in a fiber optic communications network includes the steps of:receiving a plurality of optical signals from a plurality of upstreamlocations at a power equalizer, wherein the power equalizer is locatedupstream from a splitter/combiner and downstream from an optical linetermination at step 1602; equalizing the plurality of optical signalsfrom a plurality of upstream locations at the power equalizer at step1604; and transmitting the equalized signals upstream at step 1606.

Utilizing the above-described exemplary systems according to exemplaryembodiments, a method for increasing optical signal range is shown inthe flowchart of FIG. 17. Initially a method for increasing opticalsignal range in a fiber optics communications network includes the stepsof determining transmission windows and transmission powers for aplurality of upstream locations at step 1702; receiving, at a pluralityof upstream multiplexers, a plurality of optical signals from theplurality of upstream locations at step 1704; coupling into asequentially combined optical output signal the plurality of opticalsignals from the plurality of upstream locations based upon thetransmission windows at step 1706; equalizing the sequentially combinedoptical output signal based upon the transmission powers at step 1708;and transmitting an adjusted multiplexed sequentially combined opticalsignal downstream at step 1710.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. All such variations and modifications are considered to bewithin the scope and spirit of the present invention as defined by thefollowing claims. For example, if these improvements to optical signalreach in the upstream direction make the downstream direction to nowhave the shorter reach, various parts of the above described exemplaryembodiments could be used in the downstream direction. No element, act,or instruction used in the description of the present application shouldbe construed as critical or essential to the invention unless explicitlydescribed as such. Also, as used herein, the article “a” is intended toinclude one or more items.

What is claimed is:
 1. A method for equalizing optical signalscomprising the steps of: receiving a plurality of upstream opticalsignals in a passive optical network (PON), each of said plurality ofoptical signals comprising a PON burst frame transmitted from adifferent downstream transmitter; receiving schedule information foreach of said plurality of optical signals from a system scheduler,wherein said schedule information includes at least timing and powerinformation associated with said plurality of optical signals; andadjusting each of said plurality of optical signals by either amplifyingor attenuating each of said plurality of optical signals based upon saidschedule information to mitigate non-uniform signal powers of said PONburst frames.
 2. The method of claim 1, wherein said adjusting step isperformed using a micro-mechanical optical attenuator, wherein ascheduler adjusts said micro-mechanical optical attenuator based upon aknown strength of a next to be received optical signal from saidplurality of optical signals.
 3. The method of claim 1, wherein saidadjusting step is performed using parametric amplification with avariable pump laser, wherein a scheduler adjusts said parametricamplification with a variable pump laser based upon a known strength ofa next to be received optical signal from said plurality of opticalsignals.
 4. The method of claim 1, wherein said step of adjusting isperformed at a location other than at an optical line termination or acoupler/splitter in an optical communication system.
 5. A node forequalizing optical signals comprising: an input port for receiving aplurality of upstream optical signals, each of said plurality of opticalsignals comprising a PON burst frame transmitted from a differentdownstream transmitter; a scheduled power equalizer configured forreceiving schedule information for each of said plurality of opticalsignals from a system scheduler, wherein said schedule informationincludes at least timing and power information associated with saidplurality of optical signals, and an adjuster for adjusting each of saidplurality of optical signals by either amplifying or attenuating each ofsaid plurality of optical signals based upon said schedule informationto mitigate non-uniform signal powers of said PON burst frames.
 6. Thenode of claim 5, wherein said adjuster further comprises amicro-mechanical optical attenuator, and further wherein saidmicro-mechanical optical attenuator is adjusted based upon a knownstrength of a next to be received optical signal from said plurality ofoptical signals.
 7. The node of claim 5, wherein said adjuster furthercomprises a parametric amplification device with a variable pump laser,wherein said said parametric amplification is adjusted with saidvariable pump laser based upon a known strength of a next to be receivedoptical signal from said plurality of optical signals.
 8. The node ofclaim 5, wherein said node is disposed at a location other than at anoptical line termination or a coupler/splitter in an opticalcommunication system.